In the practice of Anapanasati Bhavana, how do you know what you should do next?

Now, I read the tetrads or the stages of Anapanasati once, and you know what i thought, how can something so systematically laid down at the same time be a mystery.

It's like someone showing you which cities you have to go through..but doesn't tell you how to get there or where you should make a stop and change trains.Usually in real life i just figure things out.is this what the Buddha wants us to do.To figure things out on our own.

So my question is How will i know when to change something especially regarding the different stages in Anapanasati?

1.Do you have to consciously "switch".

or

2.Are the 16 stages that you go through, do they take place naturally if all you do is just focus on the breath..is it those just focus on the breath everything will fall into place things.

Oh and also Does anyone have a good link of the tetrads and stages...something simple,a step by step guide, that can be a good guide for this practice.

...He trains himself, 'I will breathe in releasing the mind.' He trains himself, 'I will breathe out releasing the mind.'...

...He trains himself, 'I will breathe in focusing on inconstancy.' He trains himself, 'I will breathe out focusing on inconstancy.'He trains himself, 'I will breathe in focusing fading.' He trains himself, 'I will breathe out focusing on fading.'He trains himself, 'I will breathe in focusing on cessation.' He trains himself, 'I will breathe out focusing on cessation.'He trains himself, 'I will breathe in focusing on relinquishment.' He trains himself, 'I will breathe out focusing on relinquishment.'

SN 48.10 wrote:There is the case where a monk, a disciple of the noble ones, making it his object to let go, attains concentration, attains singleness of mind. Quite withdrawn from sensuality, withdrawn from unskillful mental qualities, he enters & remains in the first jhana...

I would say that it's really all going the same way.

"And how is it, bhikkhus, that by protecting oneself one protects others? By the pursuit, development, and cultivation of the four establishments of mindfulness. It is in such a way that by protecting oneself one protects others.

"And how is it, bhikkhus, that by protecting others one protects oneself? By patience, harmlessness, goodwill, and sympathy. It is in such a way that by protecting others one protects oneself.- Sedaka Sutta [SN 47.19]